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Air Pollution Linked to Heart and Brain Risks

Keith Meyers/The New York TimesA layer of smog rests over Lower Manhattan.

It may be time to start paying more attention to those local air pollution alerts.

That is the message of three new studies this week that found, collectively, that people exposed to higher levels of air pollution have a greater risk of stroke, heart attacks and cognitive deterioration.

The impact of pollution on the heart and brain was seen over both the short and the long term. One nationwide study that followed nearly 20,000 women over a decade found that breathing in levels of polluted air like those commonly found in most parts of the country greatly accelerates declines in measures of memory and attention span. Another study in Boston found that on days when concentrations of traffic pollutants went up, so did the risk of stroke. The odds climbed by more than 30 percent even on days classified by the federal air quality index as “moderate” pollution days, which is intended to correspond to a minimal danger to health.

“At levels that the Environmental Protection Agency says are safe, we’re seeing real health effects,” said Gregory A. Wellenius, an associate professor of epidemiology at Brown University and lead author of the study linking pollution to stroke. “We saw these effects within 12 to 14 hours of when pollution levels went up.”

Studying the links between pollution and health is difficult, since so many factors are involved and it is difficult to establish a direct cause-and-effect relationship. But a link between pollutants in the air and declines in cardiovascular health has been suspected since at least the 1990s, when epidemiological research suggested that breathing in tainted air drives up rates of heart disease. The possible short-term effects of pollution remained particularly unclear, with some studies showing no immediate short-term risk. And little was known about the impact of inhaling emissions and air particles on brain function and dementia.

Dr. Wellenius and his colleagues tried to better clarify the short-term impact of air pollution by studying 1,705 stroke victims admitted to Beth Israel Deaconess Medical Center in Boston from 1999 to 2008, examining medical records to obtain the precise time a stroke actually occurred. They then cross-checked with the Environmental Protection Agency’s air quality index, which rates pollution levels in six general categories, beginning with “good,” then “moderate” and, at the very worst, “hazardous.” Some earlier studies have looked at daily air pollution levels and the number of hospitalizations for ischemic stroke or heart attacks on the same day, but that practice can be deceiving, since people sometimes wait hours or even days before going to a hospital after a stroke. “We were better able to estimate the patients’ air pollution exposure at the time of their stroke,” Dr. Wellenius said.

After controlling for age, hypertension and a slew of other risk factors for stroke, the researchers found a 34 percent higher risk at times when pollution levels climbed from “good” to “moderate.” (In the Boston area, where the study was conducted, pollution levels rarely climb very high, Dr. Wellenius said.) The effect was particularly strong when the researchers looked at levels of so-called black carbon and nitrogen dioxide, two markers of pollution from traffic.

Reducing air pollution levels by just 20 percent, an “achievable” goal, Dr. Wellenius said, “would have prevented about 6,000 of the 184,000 hospitalizations for stroke in the Northeast region” in 2007 alone, he said. The results were published this week in The Archives of Internal Medicine.

In a separate report published Tuesday in The Journal of the American Medical Association, scientists at the University Paris Descartes in France helped bolster the link between short-term exposure to air pollution and cardiovascular disease. They found that a variety of common pollutants — carbon monoxide, nitrogen dioxide, sulfur dioxide and others — raised a person’s immediate risk of having a heart attack.

Breathing in pollutants may cause harm in a number of ways, the researchers noted. They may cause inflammation linked to heart disease, increase the heart rate and thicken the blood, which can cause blood clots and accelerate atherosclerosis, or hardening of the arteries.

The smallest particles of pollution, those finer than 2.5 microns in diameter — or about one-thirtieth the width of a human hair — are particularly effective at infiltrating the body, the researchers noted. There is some evidence that they can even penetrate the brain through the nasal passages, said Jennifer Weuve, the lead author of a third study, also published in The Archives of Internal Medicine, linking pollution to cognitive decline.

Dr. Weuve’s research followed 19,409 women in the United States between the ages of 70 and 81 for about a decade, looking at changes in cognition every two years. Declines in memory and executive function, including the ability to plan and make or carry out a strategy, are normal as people get older. But the study showed that women with higher levels of long-term exposure to air pollution had “significantly” faster declines in cognition than those with less exposure to pollutants.

“Cognitively speaking, this higher exposure is as if you had aged an extra two years,” said Dr. Weuve, an assistant professor at the Rush Institute for Healthy Aging at Rush University Medical Center in Chicago. That might not sound like much, she added, but if there were a treatment “that could just delay the onset of dementia by two years, that would spare the population millions of cases of disease over the next 40 years.”